The discovery that alkylene oxides rearrange to give allylic alcohols in the presence of basic lithium phosphate catalysts sparked years of industrial research aimed at improving catalyst lifetime and productivity. Allyl alcohol, the simplest allylic alcohol, is produced by isomerizing propylene oxide. Allyl alcohol is converted to useful allyl derivatives or is modified to give 1,4-butanediol and its derivatives.
Two general isomerization processes are known: the gas-phase process (see for example, U.S. Pat. Nos. 3,044,850 and 4,720,598) and the slurry-phase process (e.g., U.S. Pat. No. 3,274,121). In the gas-phase process, the alkylene oxide is passed through supported or unsupported lithium phosphate at elevated temperatures, and the allylic alcohol is recovered and purified by distillation. A major drawback of the gas-phase process is that nonvolatile by-products accumulate on the catalyst surface over time and rapidly stifle catalyst activity.
The slurry-phase process, which is practiced commercially, was developed to overcome the catalyst deactivation problems of the gas-phase process. In the slurry-phase process, lithium phosphate is suspended in a high-boiling oil. During the reaction, a portion of the catalyst suspension is continuously removed and centrifuged to separate the tar-containing oil from the catalyst. Tars are distilled from the oil, the lithium phosphate is washed with acetone, and the purified catalyst components are recycled to the reactor. Major problems with the slurry-phase process include catalyst loss and high oil consumption (about 15 pounds of oil per 1000 pounds of allyl alcohol produced).
Low catalyst productivity is a problem common to both the gas- and slurry-phase isomerization processes. In spite of many efforts to improve productivity by varying the catalyst preparation method or process conditions, only marginal productivity improvements have resulted. The best processes known have catalyst productivities less than or equal to about 1.5 kilograms of allylic alcohol produced per kilogram of lithium phosphate per hour (see, for example, U.S. Pat. No. 3,274,121).
U.S. Pat. No. 4,720,598 teaches that high surface area lithium phosphate supported on .alpha.-alumina is a effective catalyst for gas-phase isomerization of propylene oxide to allyl alcohol. The reference stresses the need to slowly combine lithium hydroxide and phosphate salts during lithium phosphate preparation. At high lithium phosphate loading (78 wt.% Li.sub.3 PO.sub.4, 22 wt.% .alpha.-alumina) conversion of propylene oxide is 50-60%. Selectivity to allyl alcohol is good (88%), but the low productivity achieved (about 1 kg allyl alcohol per kg lithium phosphate per hour) is comparable to what can be achieved with the slurry process. Catalysts with lower lithium phosphate loadings are not shown, and the reference is silent regarding the amounts of 1-propanol and other by-products generated. 1-Propanol is extremely difficult to separate from allyl alcohol by distillation because both compounds boil at about 97.degree. C.
Improved isomerization catalysts are needed. Preferably, the catalysts are useful in a gas-phase isomerization process, since the gas-phase process is simpler and eliminates the problems of treatment and disposal of contaminated oil. A high-activity catalyst and economical process for making allyl alcohol from propylene oxide at high production rates with good selectivity to allyl alcohol is especially needed. A catalyst that has a long lifetime and is capable of regeneration is desirable.